1. Field of the Invention
The present invention relates to optical information-recording media for which recording is performed by changing, through treating with a laser beam, the reflectivity or the optical phase of a recording film, and relates in particular to a phase-change optical disk. The present invention also relates to an optical information-recording/reproduction apparatus that employs optical information-recording media to record and reproduce pieces of optical information.
2. Related Background Art
To perform information recording and reproduction using a phase-change optical disk, a laser beam is used to treat a recording film, which produces reversible state changes between a crystal state and an amorphous state, and to change the reflectivity or the optical phase of the recording film. To prepare an optical disk for recording pieces of information, immediately after the optical disk has been manufactured, a recording film, which is in the amorphous state, is treated and crystallized with a laser beam (this step is called initialization). Then, the recording film is again treated with a laser beam and is returned to the amorphous state. Films of GeTe, SbTe, GeSbTe, InSbTe and AgInSbTe are typical, well-known recording films.
To increase the recording capacity of a phase-change optical disk, makers have employed a signal processing technique, a land groove recording technique, for recording pieces of information both in a guide groove used for tracking a substrate and in a flat portion defined between guide grooves, and a super resolution reproduction technique, for enabling the reproduction of a mark that is smaller than an optical diffraction limit. In addition to these techniques, a multi-layer disk, for which more recording faces are provided on the same laser incident face, is a preferred choice for increasing the recording capacity. Consequently, since the capacity of a double layer disk that has two recording film layers can be greatly increased, the development of the double layer disk has been actively studied.
When compared with a single layer disk, the recording capacity of a double layer disk may be increased about twice, at the maximum. Actually, for a DVD-ROM that employs a red semiconductor laser, single layer 4.7-GB disks and double layer 9-GB disks, which are nearly twice in capacity, are presently being sold on the market. For a double layer disk employing the same laser incident face, as is shown in FIG. 2, a disk near the laser incident face is defined as an L0 disk 10, and a disk farther from the laser incident face is defined as an L1 disk 20. In the following explanation, the L0 disk 10 and the L1 disk 20 are also referred to as layers L0 and L1. For these layers L0 and L1, dielectric films 12 and 22, recording films 13a and 23a, dielectric films 14 and 24 and reflection films 15 and 25 are deposited respectively on and under substrates 11 and 21. The layers 10 and 20 are coupled by bonding together the reflection film 15 of the layer L0 and the dielectric film 22 of the layer L1 via an intermediate layer 16. For the reproduction of a piece of information recorded on the layer L1, the layer L0 must have a specific transmission factor relative to a semiconductor laser that is used for information reproduction. Assuming that the transmission factor of the layer L0 is defined as T0 and the reflectivity of the layer L1 itself is defined as R1, the effective reflectivity of the layer L1 would be reduced to T02×R1 at the time of information reproduction for the L1 disk 20 of a double layer disk. Assuming that T0 is defined as 0.3 (30%), the effective reflectivity of the layer L1 would be lowered to 9% of the reflectivity of the L1 disk 20 itself.
In order to obtain an effective reflectivity for the L1 disk 20, the transmission factor of the L0 disk 10 must be about 0.5 (50%). If the transmission factor of the L0 disk 10 were too high, the reflectivity of the L0 disk 10 would be reduced and the quality of signals recorded on the L0 disk 10 would be deteriorated. Therefore, in order to obtain satisfactory characteristics for both the L0 disk 10 and the L1 disk 20, preferably, the transmission factor of the L0 disk 10 be about 0.5.
As a recording film consisting of an example combination of elements, an InGeSbTe recording film has been described for the purpose of increasing the number of rewriting operations in patent document 1 (Japanese Patent Laid-Open Publication No. Hei 7-223372). In this document, In is not added to displace Sb, but is added to displace a part of GeSbTe that is the parent composition, i.e., In is added in a style of (GeSbTe)1−yIny (see patent document 1). Further, while in patent document 1 it is stated that a range of 0.03≦y≦0.3 is appropriate, as will be described later, the recording characteristics can be remarkably improved by adding a smaller amount of In to displace part of Sb with In.
Since the recording films 13a and 23a used for the phase-change optical disk have a specific absorption factor relative to a laser beam emitted for information recording and reproduction, the thicknesses of the recording films 13 and 23a must be reduced in order to obtain a transmission factor of about 0.5. Generally, the thickness of a phase-change recording film used for a single layer optical disk is set within a range of from 13 to 25 nm. On the other hand, to obtain a transmission factor of about 0.5, conventionally, the thicknesses of the recording films 13a and 23a must be reduced to about 6 nm. However, when the thicknesses of the recording films are reduced to about 6 nm, the crystallization speed is lowered, giving rise to another problem, e.g., the erasing function is degraded, or the number of rewriting operations in which a piece of information can be rewritten is reduced.
Conventionally, when the thicknesses of the recording films are reduced, a surface layer made, for example, of GeN or SiN is additionally deposited between the recording film 13a or 23a and the dielectric layers 12 and 14 or 22 and 24 to prevent the deterioration of the erasing function. However, the addition of the surface layer complicates a disk manufacturing process, and increases disk-manufacturing costs.